Traditionally, conventional onsite wastewater treatment systems (OWTSs) have consisted primarily of a septic tank and a soil absorption field, also known as a subsurface wastewater infiltration system (SWIS). Conventional systems work well if they are: installed in areas with appropriate soils and hydraulic capacities; designed to treat the incoming waste load to meet public health, ground water, and surface water performance standards; installed properly; and maintained to ensure long-term performance. These criteria, however, are often not met.
Over the past century developing countries have witnessed an explosion in sewage treatment technology and widespread adoption of centralized wastewater collection and treatment services. Scientists, engineers, and manufacturers in the onsite wastewater treatment industry have also developed a wide range of alternative technologies designed to address the shortcomings of traditional conventional systems as well as increased hydraulic loads and water contamination. These “alternative” onsite treatment technologies are more complex than conventional systems and incorporate pumps, recirculation piping, aeration, and other features. As such, alternative technologies are applied to the treatment train beyond the septic tank and often provide environments (e.g., recirculating sand filters, peat-based systems, package aeration units) that promote additional biological treatment.
Accurate characterization of raw wastewater, including daily volumes, rates of flow, and associated pollutant load, is critical for effective alternative treatment system design. Determining treatment system performance requirements, selecting appropriate treatment processes, designing the alternative treatment system, and operating the system depends on an accurate assessment of the wastewater to be treated and the effluent quality desired.
There are basically two types of onsite wastewater systems—residential and nonresidential. The required hydraulic capacity for an OWTS can be determined initially from an estimated wastewater flow. For example, the average daily wastewater flow from typical residential dwellings can be estimated from indoor water use in the home. However, maximum and minimum flows, as well as instantaneous peak flow variations, are necessary factors in properly sizing and designing system components. Alternative onsite treatment system designs vary considerably and are based largely on the type of establishment under consideration. Therefore, reliable data on existing and projected flows must be used if onsite systems are to be designed properly and cost-effectively. Accurate wastewater characterization data and appropriate factors of safety to minimize the possibility of system failure are required elements of a successful alternative wastewater system design. All OWTSs should be designed to accept and process hydraulic flows from residential or nonresidential wastewaters while providing necessary pollutant removal efficiency to achieve performance goals.
The three primary components of a conventional OWTS are: the soil beneath the SWIS; the SWIS (also called a leach field, drain field or infiltration trench); and, the septic tank. The SWIS is the interface between the engineered system components and the receiving ground water environment. SWISs provide both dispersal and treatment of the applied wastewater. Typically, wastewater is transported from the infiltration system through several different soil zones, which can act as fixed-film bisectors, where oxygen in the soil may or may not satisfy the oxygen demand generated by the microorganisms degrading the treated wastewater. If sufficient oxygen is not present, the aerobic metabolic processes of the microorganisms (biomass) can be reduced or halted and both treatment and infiltration of the wastewater can be adversely affected.
The method and pattern of wastewater distribution in a SWIS are important design considerations. Uniform distribution aids in maintaining unsaturated flow below the infiltration surface which results in wastewater retention times in the soil that are sufficiently long to effect treatment and promote subsoil reaeration. As a result, uniform distribution design can provide more complete utilization of the infiltration surface.
While many different SWIS designs and configurations are used, all incorporate soil infiltrative surfaces that are located in buried excavations. Typically, a SWIS utilizes perforated pipe to distribute the wastewater over the infiltration surface. A porous medium of aggregate, such as gravel or crushed rock, is often placed in the excavation below and around the perforated distribution pipe to support the pipe and spread the localized flow from the distribution pipe across the excavation cavity. However, the porous aggregate may be substituted by graveness or “aggregate-free” system components.
Gravelless systems are prominent in the United States today taking on many designs, including open-bottomed chambers, fabric-wrapped pipe, and synthetic materials such as expanded polystyrene foam chips. Many graveness systems use large-diameter corrugated plastic tubing covered with permeable nylon filter fabric not surrounded by gravel or rock. Other graveness systems use leaching chambers that consist of trenches or beds and one or more distribution pipes or open-bottomed plastic chambers.
Several different biological treatment processes exist for reducing biochemical oxygen demand (BOD) and total suspended solids (TSS) from septic tank effluent to meet higher effluent standards. The activated sludge process is an aerobic suspended-growth process that maintains a relatively high population of biomass by recycling concentrated biomass back to the treatment process. The biomass converts soluble and colloidal biodegradable organic matter and some inorganic compounds into cell mass and metabolic end products. The biomass is separated from the wastewater by settling in a clarifier and recycled or removed to a sludge handling process. Preliminary treatment to remove settleable solids and floatable materials is usually provided by a septic tank or other primary treatment devices.
Alternatively, fixed-film systems are biological treatment processes that employ a medium of natural or synthetic solid material that will support biomass on its surface and within its porous structure. At least two types of fixed-film systems have been employed—those in which the medium is held in place and stationary relative to fluid flow (trickling filter) and those in which the medium is in motion relative to the wastewater (e.g., rotating biological disk). A third system, which is the focus of the present invention, includes dual-process systems that encompass both fixed and suspended biomass together or in series.
The state of the art with respect to the present invention is presented in U.S. E.P.A. Office of Water, Office of Research and Development, “Onsite Wastewater Treatment Systems Manual” (February 2002) which is hereby incorporated by reference in its entirety and restated, in part, above. Importantly, improvements can be made to suspended-growth processes, including adding surfaces where biomass can attach and grow, such that the system can be categorized as a dual-process or fixed-film/suspended growth system. The present invention provides an improved fixed-film/suspended, nominally zero-net growth onsite wastewater treatment technology that is incorporated with gravel or graveness SWISs. The coupled contact aeration or controlled biomass system of the present invention is, preferably, preceded by a septic tank and followed by a aggregate or aggregate-free infiltration surface such that a fixed film of biomass can attach and grow on a medium to augment a suspended microbial population thereby providing more biomass to feed on wastewater constituents. Advantages of the well-controlled system of the present invention include increased active microbial mass per unit volume, enhanced potential for nitrification, reduced sludge production, and resilience under variable influent conditions without the need for biomass recycle. Such a controlled biomass system, as the present invention provides, can also be a low-cost means of upgrading existing overloaded OWTSs that do not currently meet BOD or nitrification goals.